The aggregate data suggest that the critical mechanism for the development of nutritional rickets is reduction in availability of calcium to the skeleton, which is largely determined by vitamin D status and intestinal calcium absorption. Our proposal focuses on the establishment of a workable definition of vitamin D deficiency in an underserved and highly vulnerable population and to assess the impact of genetic variance in VDR and DBP as factors to be considered in the recommendation of vitamin D status assessment, taking into account the outcome of 25-OHD level, and in additional studies, potential functional consequences of vitamin D related to both its classical and non-classical effects.